Application of S-PIV for Investigation of Round and Shaped Film Cooling Holes at High Density Ratios

Hot section turbine engine components are often cooled through the use of a cool film of air on the component wall. The source of the air used for film cooling comes from the compressor of the gas turbine engine and may be 800°C, or more, cooler than the hot gas path air. The temperature differential between the hot mainstream gas and the film coolant results in a large difference in density between the two gases. In order to investigate the effect of high density ratios on film cooling performance, a traditional, round hole (θ = 30°) and a laidback, fan shaped hole (θ = 30°, α = γ = 10°) were observed using Stereo-Particle Image Velocimetry (S-PIV). Flowfield measurements were performed on various planes downstream of the film cooling hole (x/d = 0, 1, 3 and 10 for the round hole and x/d = 0, 3, and 10 for the shaped hole). At each location the coolant-to-mainstream interaction was captured at multiple density ratios (DR = 1, 2, 3, 4) and blowing ratios (M = 0.5, 1.0, 1.5). Using S-PIV, the three-dimensional flow field was measured. Distributions of the flow vorticity were derived from the high speed velocity measurements taken during S-PIV testing. For the simple angle, round holes, the results show at the elevated density ratios, the coolant spreads laterally near the hole; while at DR = 1, the coolant trace is limited to the width of the film cooling hole. Furthermore, as the cooling jet exits from the round hole, the vorticity within the jet is very strong, leading to increased mixing with the mainstream. However, as the density ratio increases (at a given blowing ratio), this mixing was reduced. For a given flow condition, the rotation was reduced with the jet exiting the shaped hole (compared to the round hole), and this led to enhanced protection on the surface. While investigating both round and shaped holes, it was shown the S-PIV method is a valuable tool to observe and quantify the jet–to–mainstream interactions near the film cooled surface.